This invention relates to coating systems suitable for protecting components exposed to high-temperature environments, such as the hot gas flow path through a gas turbine engine. More particularly, this invention is directed to a coating composition that exhibits improved high temperature stability when used to protect a silicon-containing substrate.
Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. While nickel, cobalt and iron-base superalloys have found wide use for components throughout gas turbine engines, alternative materials have been proposed. In particular, silicon-based non-oxide ceramics, most notably with silicon carbide (SiC) as a matrix and/or as a reinforcing material, are candidates for high temperature applications, such as combustor liners, vanes, shrouds, airfoils, and other hot section components of gas turbine engines. Components in many of these applications are in contact with highly corrosive and oxidative environments. It has been determined that Si-based ceramics lose mass and recede at high temperatures in water-containing environments because of the formation of volatile silicon hydroxide (Si(OH)4). The recession rate due to volatilization or corrosion can be sufficiently high to require an external coating with high resistance to such environments.
Stability is a critical requirement of a coating system for a Si-based material in high temperature environments containing water vapors. Other important properties for the coating material include low thermal conductivity, a coefficient of thermal expansion (CTE) compatible with the Si-based ceramic material, low permeability to oxidants, and chemical compatibility with the Si-based material and a silica scale that forms from oxidation. As such, protective coatings for gas turbine engine components formed of Si-based materials have been termed environmental barrier coatings (EBC).
Barium-strontium-aluminosilicates (BSAS; (Ba1-xSrx)O—Al2O3—SiO2) and other alkaline earth aluminosilicates have been proposed as protective coatings for Si-based materials in view of their excellent environmental protection properties and low thermal conductivity. For example, U.S. Pat. Nos. 6,254,935, 6,352,790, 6,365,288, 6,387,456, and 6,410,148 to Eaton et al. disclose the use of BSAS and alkaline earth aluminosilicates as outer protective coatings for Si-based substrates. Of these, all but Patent No. 6,352,790 disclose stoichiometric BSAS (molar ratio: 0.75BaO.0.25SrO.Al2O3.2SiO2; molar percent: 18.75BaO.6.25SrO.25Al2O3.50SiO2) as the preferred alkaline earth aluminosilicate composition, with layers of silicon and mullite (3Al2O3.2SiO2) employed as bond coats. The BSAS coatings are typically produced by air plasma spraying (APS) followed by heat treatment to contain at least 50% of the crystalline celsian phase, which is stoichiometric BSAS. U.S. Pat. Nos. 6,254,935 and 6,365,288 further teach that the coating contains at least 80% of crystalline celsian and hexacelsian phases, which differ crystallographically but have the same stoichiometric BSAS chemistry (the term “stoichiometric phase” will be used in reference to the presence of both the celsian and hexacelsian phases, though the celsian phase is preferred). As such, each of these patents allows for the presence of significant amounts of non-stoichiometric phases in addition to the stoichiometric celsian and hexacelsian phases.
Notwithstanding the above-noted advances, further improvements in coating life are required. In particular, longer exposures at temperatures sustained in the combustion environment of a gas turbine engine (e.g., above 2300° F. (about 1260° C.) combined with high pressure steam and high gas velocities) have resulted in the volatilization of existing BSAS materials, causing coating recession that ultimately leads to degradation of the environmental protective properties of the coating. In order for Si-based materials to be suitable for more demanding aircraft engine applications such as vanes, blades and combustors, coatings will be required that exhibit lower recession rates.